Is there a way in rust to have a non-static non-const global variable?

Question

Is it possible to have a mutable non-static global variable in Rust?

Say I wanted this:

pub let foo: u64 = 47;

This will not work, as rust does not allow the let keyword outside of a function.

If it's not `const` and not `static`, what it is? What will be the difference? — Chayim Friedman, Apr 12 '22 at 12:03
"Say I wanted this" why? What is that supposed to achieve? `let` is a way to declare a set of block-local variables, a global is by definition not in a block. — Masklinn, Apr 12 '22 at 12:37
Does https://stackoverflow.com/questions/22752775/keeping-a-variable-alive-across-multiple-function-calls-in-rust help? — hkBst, Apr 12 '22 at 15:03

CATboardBETA · Answer 1 · 2022-04-12T12:04:55.420

-5

Create a container structure:

pub struct FooContainer {
    pub foo: u64,
}

pub const foo: FooContainer = FooContainer { foo: 47 };

You can now access it via

mod foovar;

pub main() {
    println!("{}", foovar::foo.foo)
}

Your instantiation of the struct can remain const since it's field foo is still mut.

edited Apr 12 '22 at 12:04

answered Apr 12 '22 at 11:58

CATboardBETA

Now it does not raise a syntax error but [just prints 47 twice](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=832059bc21ab488dad3733591d9cd343) (and raises a warning for trying to mutate a constant, and correctly so). – Chayim Friedman Apr 12 '22 at 12:07
1

@CATboardBETA you seem to be deeply misunderstanding the behaviour of `const`. Semantically, the `const` value is copied to its point of use (think of it as a const always being inlined, and behaving such as far as the compiler is concerned). So you can assign to the const item because in reality a copy is instantiated locally, its attribute is updated, then the copy is dropped. That has no effect. Or at least not that of being a shared mutable global, assuming that's what you were looking for. – Masklinn Apr 12 '22 at 12:45
1

This doesn't work. If you're going to try to do a self-answer, at least make sure you test it first to see if actually does what you think. – Herohtar Apr 12 '22 at 15:12

1 Answers1